Artificial Ionospheric Irregularities Research Articles

Application of an imaging HF riometer for the observation of scintillations of discrete cosmic sources

Scintillations of discrete cosmic radio sources have been investigated theoretically and experimentally for radiation scattered from natural and artificial inhomogeneities in the auroral zone ionosphere. The observations were performed at a frequency of 38.2 MHz with the multibeam phased array antenna of the imaging riometer located at Poker Flat, Alaska. For quiet ionospheric conditions and weak scintillations, the spatial spectrum and transverse motion velocities of the Fresnel inhomogeneities have been recovered. A theoretically based approach has been developed to estimate the time coherence interval of the radiation for the case of saturated scintillations and strong ionospheric turbulence. Examples of scintillation records are analyzed for the case of scattering from artificial ionospheric irregularities induced by high‐power HF radiation from the high‐power auroral simulation (HIPAS) ionospheric modification facility. The effect of aspect‐sensitive scattering of the cosmic source radiation by artificial field‐aligned small‐scale irregularities has been predicted and detected. Prospects for routine observations of the effects of scintillations and aspect‐sensitive scattering are discussed for special operation modes of the ionospheric modification facility.

On specific features of diurnal variations in characteristics of the diagnostic stimulated electromagnetic emission and their relation to the evolution of artificial ionospheric irregularities

We analyze variations in characteristics of the diagnostic stimulated electromagnetic emission of the ionosphere in the evening hours including the times of sunset both on the Earth’s surface at the observation point and in the ionosphere over it. It is found that an increase in typical times of evolution of the diagnostic emission begins to be recorded just before the sunset on the Earth’s surface when the ionosphere is illuminated and its parameters are not significantly changed yet. We state that the typical times of evolution of the diagnostic emission increase when the pump-wave frequency approaches the critical frequency of the ionospheric F2 layer, but such an effect is not as significant as when passing from the illuminated to the unilluminated ionosphere. It is established that at the stage of diagnostic sounding the pump-wave pulse power does not exert any notable influence on the first (fast) stage of relaxation of small-scale artificial ionospheric irregularities, but can increase the decay time of the irregularities at the second (slow) stage of relaxation. Capabilities of the method for a study of artificial plasma turbulence using the diagnostic stimulated electromagnetic emission are discussed.

Some distinctive features in the behavior of small-scale artificial ionospheric irregularities at mid-and high latitudes

We present the results of experimental studies of some features in the behavior of small-scale artificial irregularities (SSAIs) at mid-and high latitudes based on the “Sura” and EISCAT/HEATING HF facilities. Observations were performed by the method of aspect scattering using a network of diagnostic paths having a common reception point located near St. Petersburg. We found that an extremely long duration of the second (slow) stage of SSAI relaxation of up to 5 min occurs in the evening hours when the ionosphere above the “Sura” facility is illuminated by the Sun, but the solar terminator travels through the magnetically conjugated ionosphere. The conjecture is made that the processes initiated by the terminator are mostly responsible for secondary ionospheric turbulence maintaining the irregularities above “Sura.” A drastic increase in the Doppler spectra width of the scattered signals is revealed when the magnetically conjugate point of the ionosphere is located on the shade side of the terminator, but the ionosphere above the “Sura” facility is still lighted. It is assumed that the “ run away” of photoelectrons from the day to the night side could reduce the threshold of excitation of artificial irregularities, leading to an increase in their intensity. The presence of fairly intense scattered signals was detected from the “Sura” and EISCAT/HEATING experimental results both under conditions of pulsed HF heating after continuous heater-on periods and cycled HF heating by short pulses. In the case of pulsed heating by short pulses with duration τp < 100 ms and average radiated power Pa below the threshold power Pthr of the SSAI generation cutoff the irregularities can be maintained due only to striction parametric instabilities. The excitation of irregularites under the cycled HF pumping with the pulse duration τp = 384 ms for Pa comparable with Pthr was detected. The aspect-angle dependence, or the so-called magnetic zenith effect, was found in the SSAI intensity. The residual turbulence aftereffects played a significant role in the SSAI development.

Determining the ionospheric irregularity velocity vector based on doppler measurements in the artificially modified F 2 region of the polar ionosphere

The method for estimating the behavior of the ionospheric irregularity motion vector in the artificially disturbed HF ionospheric region has been proposed, and this behavior has been analyzed based on the simultaneous Doppler observations performed on several paths using the method of bi-static backscatter of diagnostic HF signals by small-scale artificial ionospheric irregularities. The Doppler measurements were performed during the modification of the auroral ionosphere by powerful HF radiowaves emitted by the EISCAT heating facility (Tromso, Norway). It has been obtained that the dynamics of the ionospheric irregularity directions in the F region, calculated based on the Doppler measurements of the total vector of the ionospheric irregularity velocity above the Tromso EISCAT radar at a frequency of 931 MHz, is in satisfactory agreement with such calculations performed using the three-position method.

Features of Excitation of Stimulated Electromagnetic Emission of the Ionosphere Modified by an Oblique High-Power Radio Wave

We present the results of measuring the characteristics of the stimulated electromagnetic emission (SEE) of the ionosphere with variation in the zenith angle of a pump beam of high-power O-mode radio waves in the geomagnetic-meridian plane. The experiments were performed at the midlatitude heating facility “Sura.” It is established that the maximum intensity of the DM and BC components of SEE is observed for southward inclination angles θ ≈ 8°--12° of the antenna beam, for which the most intense generation of artificial small-scale ionospheric irregularities also takes place. Based on the results of measurements near the fourth and fifth harmonics of the electron gyrofrequency, it is found that the first component of the BUM (BUM-1) is generated only when the pump wave reaches the plasma-resonance region. This allows one to assume that, unlike the second component of the BUM (BUM-2), whose generation is determined by development of instability in the upper-hybrid resonance region, the BUM-1 generation mechanism should be related to processes of interaction between a high-power radio wave and the plasma in the plasma-resonance region.

Experimental Studies of the Effects Observed During the Nonlinear Interaction of Two High-Power Radio Waves in a Magnetoplasma

We present the results of experiments on modification of the ionospheric F region by two high-power (Peff ≈ 20 MW) O-mode electromagnetic waves. The experiments were performed at the “Sura” heating facility of the Radiophysical Research Institute (Nizhny Novgorod, Russia) in May 23–27, 1993 at the pump frequencies near the 4th, 5th, or 6th harmonics of the electron gyrofrequency. Ionospheric perturbations were diagnosed by measuring the stationary spectral characteristics of the stimulated electromagnetic emission (SEE) of the ionospheric plasma. We determine the features of variation in the spectral characteristics of particular SEE components during the simultaneous heating of the ionospheric plasma by two radio waves in comparison with the case of a monochromatic pump wave. We observed the effect of enhanced generation of the broad up-shifted maximum (BUM) by the higher-frequency pump wave. This is accompanied by strong suppression of the BUM induced by the lower-frequency pump wave. It is shown that the effects observed during the two-frequency heating of the ionosphere have well pronounced gyroharmonic properties, i.e., depend on both the electron-gyroharmonic number and the frequency detuning of the pump waves from a harmonic of the electron gyrofrequency. We also pointed out that a change in the properties of artificial small-scale ionospheric irregularities (striations) excited by high-power radio waves is not the cause of a change in the properties of the down-shifted maximum and the BUM during the two-frequency modification of the ionospheric plasma. Ways for the further development of these studies are discussed.

Doppler shift simulation of scattered HF signals during the Tromsø HF pumping experiment on 16 February 1996

Abstract. Comparisons between bistatic scatter measurements and simulation results during the Tromsø HF pumping experiment on 16 February 1996 are made. Doppler measurements of an HF diagnostic signal scattered from the field-aligned irregularities (FAIs) in the auroral E-region were carried out on the London – Tromsø – St. Petersburg path at 9410 kHz from 21:00 to 22:00 UT. The scattered signals were observed both from natural and artificial ionospheric irregularities located in the vicinity of Tromsø. To simulate the Doppler frequency shifts, fd , of scattered signals, a radio channel model, named CONE, was developed. The model allows for ray tracing, group and phase paths, and Doppler frequency shift calculations. The calculated Doppler shifts were analyzed for dependence on the magnitude and direction of plasma velocities in the scattering volume. It was found that the velocity components in the north-south direction are crucial for explaining the Doppler frequency shifts of the scattered diagnostic signals. To simulate fd , real velocities obtained from the EISCAT UHF radar at an altitude of 278 km and from the digital all-sky imager during the experiment were employed. The simulation results of Doppler frequency shift variations with time are in reasonable agreement with the experimental Doppler shifts of scattered signals on the London – Tromsø – St. Petersburg path.Key words. Ionosphere (active experiments; ionospheric irregularities) Radio science (ionospheric propagation)

Excitation of an Additional Region of Short-Wavelength Plasma Oscillations in Ionospheric Heating Experiments

We discuss transfer of plasma waves, excited by a powerful radio wave due to its scattering on artificial ionospheric irregularities, into an additional region of very short plasma oscillations polarized almost perpendicular to the magnetic field. Such a region can exist in the magnetized ionospheric plasma due to the strong spatial dispersion. We take into account the plasma-wave diffusion over the spectrum caused by multiple scattering on irregularities, as well as the nonlinear process of plasma-wave interaction due to induced scattering by ions. The latter process leads to the transfer of primary plasma waves into the additional region. The induced scattering is considered in the differential approximation valid for sufficiently smooth plasma-wave spectra. The numerical calculations are performed for a Maxwellian plasma in which suprathermal electrons are absent. It is shown that in this case, the additional region of plasma waves is excited if the pump frequency is close to but slightly less than the fourth electron gyroharmonic, so that the absorption of primarily excited plasma waves becomes sufficiently strong. Application of our calculations to the results of ionospheric experiments is discussed.

Spatial Spectrum of Artificial Ionospheric Irregularities Induced by Powerful HF Radiowaves

We present measurement results and spatial-spectrum shapes of the dependence of the relaxation time on the scale across the geomagnetic field for artificial ionospheric irregularities (AIIs) induced in the upper ionosphere by powerful HF radiation of the SURA heating facility. The irregularity diagnostics was based on observing amplitude scintillations of a 243-MHz beacon signal from a quasi-geostationary, solar-synchronous satellite and on measuring field-aligned scattering at frequencies 15 and 20 MHz. The satellite signal was received at the Kazan State University Observatory. The field-aligned scattering signals were received and analyzed by a bistatic HF radar based on the UTR-2 radio telescope located near Kharkov (Ukraine). It is shown that irregularities of the electron density, whose two-dimensional spectrum in the plane perpendicular to the geomagnetic field is the power law ae⊥-p with index p≲2, are developed in the scale range 30–60 ≲ l⊥ ≲ 200–400 m. In this case, the relative fluctuations (〈δN2〉)1/2 of the electron density increase with decreasing scale l⊥=2π/ae⊥. The estimate 〈δ N2〉)1/2 ≲ 1–1.5% is obtained for a heating power of 150 MW and irregularity scales l⊥ ∼ 30–60 m at which the fluctuations are maximum. The measured dependence of the AII relaxation time, defined as the e-folding time of the amplitude-scintillation intensity, has the form τr ∝ l⊥α. If l⊥ ≲ 30–60 m, then the index α is close to 2, whereas the effective diffusion coefficient D⊥ ≃ (2–3)· 10-1 m2/s corresponds to the ambipolar cross-field diffusion coefficient in a magnetized plasma. The time τr for scales l⊥≳ 60–100 m is independent of l⊥ and increases with decreasing velocity of regular drift of the plasma. The Doppler-spectrum broadening (〈Δν2〉)1/2 ≲ 0.6 Hz observed when receiving field-aligned scattered signals can be related to chaotic motions of plasma-density disturbances whose random drift velocities amount to (〈Δv2〉)1/2 ≃ 2–3 m/s for scales l⊥ ∼ 20 m at which the power-law index changes drastically.

Interaction between artificial ionospheric irregularities and natural MHD waves

This paper concerns the interaction between small‐scale artificial ionospheric inhomogeneities and large‐scale natural MHD waves (geomagnetic pulsations). The inhomogeneities were produced at F region altitudes by powerful HF radiation from the Sura heating facility in Russia. Diagnostics were provided by resonant scattering of decametric radio waves emitted by several broadcast and dedicated transmitters. The scattered waves were received at the UTR‐2 radio telescope in Ukraine with a multichannel coherent receiver equipped with highly directional phased antenna arrays. This instrument allowed us to investigate spatial and temporal variations in intensity and frequency of the signals scattered by the inhomogeneities with the scale size of l ≃ 10 m. Previous investigations have shown the existence of quasi‐sinusoidal variations in the signal frequency associated with the E × B plasma drift due to the MHD wave. In this study, we investigate similar periodicities in the signal intensity, which in some cases were observed simultaneously with the frequency variations. However, unlike the frequency variations, the intensity oscillations appeared to be considerably nonuniform within the pumped region of the ionosphere with a horizontal scale size of L ≃ 100 km. We discuss different explanations for the detected effect and suggest that its most probable cause is a transformation of the MHD wave energy into the energy of the inhomogeneities.

Influence of small-scale irregularities on the features of the overshoot effect in the temporal evolution of stimulated electromagnetic emission. Part I. Development stage

We present the results of experimental studies of the evolution of stimulated electromagnetic emission of the ionosphere (SEE) under the F-layer modification by powerful HF radio waves in a broad range of the pump wave frequencies. We compare the parameters of the overshoot effect in SEE evolution to the observations of anomalous attenuation and field-aligned scattering of radio waves. We show the overshoot effect to result from the anomalous attenuation under scattering at artificial small-scale ionospheric irregularities. We found the characteristic time scales of the overshoot effect development to decrease and its value to increase as the pump wave frequency decreases from 6 to 4 MHz; this is attributed to the observed increase of the irregularities amplitude at scales\(l \bot \sim 3 - 10m\) m.

Generation of artificial ionospheric plasma density irregularities with prescribed spectra

The creation of artificial plasma density irregularities (AI) with prescribed spectra in the ionospheric heating experiments is discussed. We show that periodic successions of powerful pulses, pumped into the F-region of the ionosphere lead to obtaining AI with the controlled stationary spatial spectrum and allow us to change power low spectral index 2β at least from 2 β = 2.2 to 2 β = 3.4 by changing of the powerful pulse time schedule.

Coordinated study of subkilometer and 3‐m irregularities in the F region generated by high‐power HF heating at Arecibo

High‐power high‐frequency transmitters near Arecibo were used to generate artificial ionospheric irregularities in the F region. Radio star scintillation observations at 430 MHz were performed at Arecibo Ionospheric Observatory with the 305‐m antenna, and radar backscatter measurements at 50 MHz were simultaneously made from Guadeloupe Island to probe the subkilometer and 3‐m irregularities in the heated volume. Scintillation studies indicate a low‐frequency modulation of the faster intensity fluctuation structure. By the use of plasma drift data this low‐frequency temporal structure translates to spatial dimensions of 1–2 km. The frequency of the modulation envelope is found to be controlled by the heater power and is related to the dominant irregularity wavelength generated by the self‐focusing instability. Scintillation spectra imply a steep power law index of −5 in the scale length range of about 300 m to 150 m and a shallow index of −2 at less than 150 m. The steep power law index may arise from an aperture averaging effect of the large 305‐m antenna at Arecibo. Simultaneous measurements of 430‐MHz scintillations and 50‐MHz radar backscatter from field‐aligned striations were performed to show that subkilometer irregularities can be generated by both O and X mode heating whereas the 3‐m irregularities are excited only by the O mode heating, as is predicted by the theories of self‐focusing and parametric instability. The width of the 50‐MHz echo Doppler spectra is observed to be very narrow, only ∼2–3 Hz, and independent of the background plasma drift, implying that the frequency bandwidth of the scattered signal is probably controlled by the instability process.

On reflexion and trapping of upper-hybrid waves

The theme of the paper is the relation between the so-called Z mode and the upper-hybrid wave in a magnetized plasma. First, certain problems as to reflexion and transmission of upper-hybrid waves at quasi cut-off (that is, cut-off predicted within electrostatic approximation) are stated and solved by singular perturbation technique. Then the theory is extended to a problem concerning generation and trapping of upper-hybrid waves in magnetic field aligned electron density irregularities. First a single density depression is considered, and then the formalism is further extended to a system of many parallel density depressions. The study was motivated by problems in the theory of artificial small-scale ionospheric irregularities occurring in radio wave experiments.

Scattering of radio waves by periodic artificial ionospheric irregularities

Scattering of radio waves by periodic artificial ionospheric irregularities

Spectra of artificial ionospheric irregularities at different heights

Spectra of artificial ionospheric irregularities at different heights